Sleeper Sharks Not So Sleepy
INTRO: As scientists seek to unravel the causes of Steller sea lion and harbor seal declines in Alaska, a number of likely culprits come to mind. Killer whales, some scientists say, may be responsible. Or perhaps changing ocean conditions are at fault. Now add to the debate a little-known shark called the Pacific sleeper shark. As Doug Schneider reports in this week's Arctic Science Journeys Radio, the sleeper shark may not be so sleepy after all.
STORY: The Pacific sleeper shark got its name because it's thought to be rather sluggish, and because they're often found just lying around on the ocean floor. Bruce Wright is a shark scientist with the National Marine Fisheries Service in Juneau, Alaska.
WRIGHT: "Pacific sleeper sharks usually live down really deep. Most people think of them as a nonaggressive shark. One of the common names for them is mud shark. When you bring them up alongside the boat, they're very docile. They're a real soft-looking shark because they don't have that big square dorsal fin. Almost every researcher I've talked too, even a lot of shark researchers, think of them as a docile and nonaggressive shark."
But recently Wright had the opportunity to see sleeper sharks in action in Alaska's Prince William Sound. Each summer, millions of salmon return to the sound to spawn in streams and hatcheries. Increasingly, thousands of sharks have been converging there to feed on the migrating salmon. Among them are sleeper sharks, which Wright says are not as docile as scientists think.
WRIGHT: "We're real interested in the increased number and size of sharks people are catching. Last year we looked at six sleeper shark stomachs and found that the sharks are eating salmon. That indicates that they are not docile, that they can be fast and aggressive predators."
Witnessing sleeper sharks easily catch fast-swimming salmon got Wright to thinking that maybe the sharks are capable of catching even larger, faster prey. He wondered if sleeper sharks might be at least partly responsible for the decline of Alaska's Steller sea lions and harbor seals.
WRIGHT: "My belief was that there is the potential for the Pacific sleeper shark to take seals and other marine mammals, and the reason I thought this might be the case is because the Greenland sleeper shark's primary prey are seals."
To find out, Wright joined researchers with the International Pacific Halibut Commission to conduct surveys last summer of halibut in the Gulf of Alaska. Researchers caught halibut on fishing lines, called longlines. Along with halibut, the researchers also caught sleeper sharks. It proved a perfect opportunity.
WRIGHT: "When the commission runs their surveys they catch a lot of sleeper sharks, and on the last leg of their survey they caught 592 sleeper sharks. These sharks ranged in size from six to seven feet, to up to 18 feet long, and there's been sleeper sharks caught in these waters that are 24 feet long. So these sleeper sharks can get almost as big as a small orca."
All but thirty sleeper sharks were released unharmed after being measured, weighed and fitted with tags. The sharks taken back to the lab allowed Wright to make a unique discovery.
WRIGHT: "Thirty sharks were sampled and of those, five of them had harbor seals and Dall's porpoise parts in their stomachs."
That's enough to make Wright wonder if maybe scientists have overlooked the sleeper shark as yet another key player in the North Pacific food chain. Wright says he hopes to obtain funding to learn more about this misunderstood predator of the sea.
WRIGHT: "For a scientist, it's like being a kid out there, because there's a million questions. Every day we were out there we were learning something new about these species, things that nobody knew at all."
OUTRO: This is Arctic Science Journeys Radio, a production of the Alaska Sea Grant Program and the University of Alaska Fairbanks. I'm Doug Schneider.
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Thanks to the following individuals for help preparing this script:
Bruce A. Wright, Executive Director
Lee Hulbert, Fisheries Research Biologist
Arctic Science Journeys is a radio service highlighting science, culture, and the environment of the circumpolar north. Produced by the Alaska Sea Grant College Program and the University of Alaska Fairbanks.
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